A meteorological radar system locates precipitation such as rain, snow or hail, measures their intensity, and possibly identifies hazardous phenomena. Most meteorological radar systems are installed on the ground and are often part of a much larger meteorological surveillance network. More and more airborne applications are seeing the light of day, however, air transport being particularly concerned with meteorological phenomena. It is notably a question of circumventing cumulonimbus clouds, which are enormous clouds of which pilots are very nervous because they sometimes give rise to violent storms. Even airliners divert their route to avoid crossing the path of certain particularly menacing cumulonimbus cloud formations. Lightning, hail, and strong wind shears in the cloud are added to the risk of icing up and can endanger the flight if the pilot attempts to fly through.
A meteorological radar system detects the large voluminous targets that clouds represent. It must give the position, the size and the speed of the elements of the cloud, and then deduce therefrom its danger level. To do this, a meteorological radar system can emit a wave in the X band, for example. The distance to the elements of the cloud is deduced from the time necessary for the emitted pulse to make the round trip between the antenna and the cloud at the speed of light. This time simply corresponds to the time period between emitting a pulse and receiving its echo. Estimating the size of a cloud entails estimating its volume, i.e. its depth and the maximum horizontal distance over which it extends, as well as its elevation, i.e. the maximum vertical distance over which it extends. The estimate of the area results from processing the azimuth scanning of the radar beam, while the estimate of the elevation results from processing the scanning in elevation of the radar beam. An airborne meteorological radar system continually scans a wide field in azimuth, the extent of which is of the order of +/−90 degrees with respect to the direction flown by the aircraft. The field scanned in elevation is smaller, typically of the order of 10 degrees for an observation distance exceeding 40 nautical miles.
Now, in the context of an airborne application, the carrier is in continuous movement. The pitch, yaw and roll angles of an aircraft vary all the time, which greatly complicates the logic of scanning in azimuth and in elevation of the meteorological volume in front of the aircraft. This is one of the technical problems that the present invention proposes to solve.
The solution most often used is to employ mechanical scanning with respect to two axes. This mechanical scanning on the one hand compensates the unintentional pitch and yaw motion of the aircraft. It also scans the forward space by effecting scans in horizontal layers for a series of elevations of the antenna beam. This solution represents a particularly severe penalty in the case of a multimode radar system. The antenna of a multimode radar system can be called upon not only to scan a large meteorological volume but also to insert supplementary measurements outside of the meteorological volume. Note that multimode scanning of the meteorological field requires a beam agility that is difficult to obtain mechanically because of the mechanical inertia of the antenna, which necessitates high motor torques, which severely tests the motors and therefore the reliability of the system. This implies overspecification of the entire system, representing a penalty as much in terms of weight as in terms of electrical power consumption and finally of cost. Based on a purely mechanical solution with two rotation axes, the inertia of the antenna can be compensated only by overspecifying the motors, which represents a penalty in the context of an airborne application. This type of motorized antenna radar system also functions by scanning the meteorological volume in horizontal layers. Consequently, for a given azimuth, there is a significant delay between measurements for the upper portion of the meteorological volume and measurements for the lower portion. As a result the measurements in a vertical slice are hardly contemporaneous, which induces an inaccuracy term into the vertical processing of the signals. One solution that could be envisaged would be to have the antenna effect mechanical scanning, but the inevitable conclusion is that, because of the small vertical extent of the field, the frequency of the antenna turnaround phases would impose a heavy penalty on the efficiency of the radar system, i.e. the ratio between the time usable by the radar function and the overall time.
There are also frequency dispersive antennas that vary the pointing direction of the beam by varying the feed frequency of the antenna. A meteorological radar system using a frequency dispersive antenna could be envisaged. However, most such antennas have a large overall size and are not well suited to onboard applications. Frequency dispersive antennas use a rear face waveguide to feed by coupling a front-face radiating waveguide, the radiating waveguide forming the antenna as such. Because of constraints linked to this coupling, the feed waveguide must be disposed behind the antenna in a plane perpendicular to the antenna, whence a large overall size. There is a solution whereby the feed waveguide is pressed flat against the back of the antenna, as described in the French patent application filed Jun. 3, 2005 and published under the number FR 2 886 773. However, this latter solution notably has the drawback of using oblique waveguides, on the front face and on the rear face. Such waveguides, the complex wave paths whereof include many diversions, can be difficult to fabricate. This impacts on the cost of the antenna, which can become prohibitive, notably in the case of a civil application such as a meteorological radar system.
Even more complex alternative solutions could be envisaged, based on electronic scanning by variable microwave phase-shifters. Used more particularly in military applications, such solutions are not well suited to meteorological radar systems. Given the low reflectivity of the voluminous targets that clouds form, a meteorological radar system necessitates secondary and diffuse lobe levels below −30 dB at the radiation maximum. To achieve such performance, a very large number of phase-shifters would be required, or a smaller number having a very low resolution quantizing function, and thus a large number of bits. Apart from the fact that power consumption would be very high, the large number of phase-shifters or the use of phase-shifters with a large number of bits would make the cost of the antenna prohibitive. Moreover, the reliability of an electronic scanning antenna using microwave phase-shifters continues to be more difficult to guarantee than that of a mechanical antenna, as secondary and diffuse lobe performance can deteriorate rapidly with failures of phase-shifters or their control circuits.